Abstract
Mössbauer spectra of samples containing nano-sized magnetic particles exhibit a transition from a magnetically split spectrum to a doublet or singlet when the temperature is varied, and zero-field-cooled magnetization measurements show a maximum at a certain temperature. When the interactions are weak these features can be explained by superparamagnetic blocking, but in the case of strong interactions they can be explained by ordering of the magnetic moments. A phase diagram, which elucidates these possibilities, is presented.